Copper-Mediated Dichotomic Borylation of Alkyne Carbonates: Stereoselective Access to (E)-1,2-Diborylated 1,3-Dienes versus Traceless Monoborylation Affording α-Hydroxyallenes

Article abstract of DOI:10.1002/anie.202014310

A mild copper-mediated protocol has been developed for borylation of alkynyl cyclic carbonates. Depending on the nature of the borylating reaction partner, either stereoselective diborylation of the propargylic surrogate takes place, providing convenient access to (E)-1,2-borylated 1,3-dienes, or traceless monoborylation occurs, which leads to α-hydroxyallenes as the principal product. The dichotomy in this borylation protocol has been scrutinized by several control experiments, illustrating that a relatively small change in the diboron(4) reagent allows for competitive alcohol-assisted protodemetalation to forge an α-hydroxyallene product under ambient conditions.

Full text of DOI:10.1002/anie.202014310

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Accepted Article
Title: Cu-mediated Dichotomic Borylation of Alkyne Carbonates:
Stereoselective Access to (E)-1,2-Diborylated 1,3-Dienes versus
Traceless Monoborylation affording α-Hydroxyallenes
Authors: Kun Guo and Arjan Willem Kleij
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10.1002/anie.202014310
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RESEARCH ARTICLE
Cu-mediated Dichotomic Borylation of Alkyne Carbonates:
Stereoselective Access to (E)-1,2-Diborylated 1,3-Dienes versus
Traceless Monoborylation affording -Hydroxyallenes
Kun Guo,[a] and Arjan W. Kleij*[a][b]
[a]
K. Guo and Prof. A. W. Kleij
Institute of Chemical Research of Catalonia (ICIQ), the Barcelona Institute of Science and Technology, Av. Països Catalans 16, 43007 - Tarragona, Spain.
E-mail: akleij@iciq.es
[b]
Prof. Dr. A. W. Kleij
Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Spain

Supporting information for this article is given via a link at the end of the document.
Abstract: A mild copper mediated borylation protocol of alkynyl cyclic
carbonates has been developed. Depending on the nature of the
borylating reaction partner, either stereoselective diborylation of the
propargylic surrogate takes place providing convenient access to (E)-
1,2-borylated 1,3-dienes, or traceless mono-borylation that leads to
-hydroxyallenes as the principal product. The dichotomy in this
borylation protocol has been scrutinized by several control
experiments illustrating that a relatively small change in the diboron(4)
reagent allows for competitive alcohol-assisted protodemetalation to
forge an -hydroxyallene product under ambient conditions.
transformations has significantly expanded over the last five years.
In particular, we found that Cu-catalyzed decarboxylative
silylation of cyclic carbonates functionalized with alkyne groups is
feasible affording a library of tetrasubstituted 2,3-allenols.[9]
Organoboronate compounds play a privileged role in organic
synthesis representing benchmark-stable synthons featuring CB
bonds that can be utilized to create a wide range of complex
molecules.[1] In particular, bis(pinacolato)diboron (B2pin2) and its
congeners are versatile borylating agents which have enabled the
preparation of a great diversity of (un)saturated boron derivatives
including allyl, allenyl, propargyl, vinyl and alkyl/aryl boronates
through well-established catalytic carbon-boron bond formation
processes.[2] Many elegant strategies have been developed over
the years, and in this context the borylation of propargyl
substrates displays a particular diversity of reactivity patterns that
can be controlled by an appropriate transition metal catalyst.
Pioneering work reported by Ito and Sawamura in 2008
illustrated the regio- and stereoselective borylation of linear
propargyl carbonates under Cu catalysis affording a variety of
mono-borylated, tri- and tetrasubstituted allenes.[3] In 2017,
Tortosa et al. demonstrated that propargylic epoxides can be
conveniently transformed into their formally reduced -
hydroxyallenes by Cu-catalysis in the presence of B2pin2 and
methanol (Scheme 1a),[4a] with a syn-elimination of the Bpin group
playing a pivotal role in this process. The combination of B2pin2
and an alcohol as a reducing medium mimics the utilization of
copper hydride chemistry reported independently by Krause
(Scheme 1b)[5] and Ito/Sawamura (Scheme 1c).[6] Both groups
demonstrated that Cu-mediated formal reduction of propargylic
epoxides and propargylic linear carbonates could be
accomplished, respectively, by PMHS (polymethylhydrido-
Scheme 1. Formal reduction of propargylic substrates (a-c) and dichotomic
reactivity observed in the Cu-mediated conversion of alkynyl cyclic carbonates
in the presence of borylating reagents (This Work).
Inspired by the pioneering work described in Scheme 1[4a,5-6]
and the structural versatility of alkynyl cyclic carbonates,[8a-e,9] we
envisioned that borylation of a new type of propargylic surrogate
could significantly expand the diversity of functional and highly
substituted -hydroxyallene scaffolds, which are useful building
blocks for heterocyclic chemistry and natural product synthesis.[10]
In the course of these studies, we discovered an unexpected
though unique dichotomic behavior of the borylating agent
providing selective pathways leading either to an (E)-diborylated
1,3-diene or -hydroxyallene product with the same catalytic
system. Apart from the synthetic utility of the modular alkynyl
cyclic carbonates to serve as common substrates for product
diversion, the reasons for this remarkable dichotomic reactivity
siloxane) as the stoichiometric hydride source offering
straightforward synthetic route towards allenes.
a
The exploration of the reactivity of both allylic[7] and alkynyl
cyclic carbonates and similar heterocycles[8] as modular
substrates in transition metal catalyzed stereoselective
1
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RESEARCH ARTICLE
has also been studied through a series of mechanistic control
reactions.
entry 11). The identity of the main stereoisomer (E) was confirmed
by X-ray analysis (see the inset in Table 1).[12]
First, cyclic carbonate S1 and B2pin2 were combined to
examine the influence of various reaction parameters on the
selectivity of the borylation process (Table 1). We initially pre-
screened a wide series of ligands, bases and solvents using a
high-throughput experimentation approach (see the Supporting
Information, SI). From this first screening phase, we selected the
carbene
precursor
ICy·HCl
(1,3-dicyclohexylimidazolium
hydrochloride), NaOt-Bu as base and THF/i-PrOH as medium on
the basis of the selectivity and yield (66%) for 1,2-diborylated 1,3-
diene product 1 and the overall stereoinduction (E/Z = 33:1). [11]
Table 1. Cu-mediated transformation of alkynyl cyclic carbonate S1 and B2pin2
under various reaction conditions leading to diborylated product 1.[a]
Scheme 2. Scope of 1,2-diborylated 1,3-dienes (1-16) using various alkynyl
cyclic carbonates S1-S16 and B2pin2 as reagents under the optimized catalytic
conditions (Table 1, entry 22). All yields are of the isolated product, E/Z ratios
were determined by 1H NMR.
[a] The comparative reaction conditions are shown in the scheme. If not stated
otherwise, the E/Z ratio of 1 is similar to the one observed using CuCl/ ICy·HCl
as catalyst, NaOt-Bu as base, and i-PrOH/THF as medium. [b] A complex
product mixture was obtained.
Then we set out to screen the influence of the amount of
B2pin2, base additive, solvent and reaction temperature. As can
be judged from Table 1, the reaction proved to be inadequate in
the absence of either the Cu precursor, base additive or the
carbene ligand (entries 1-3). The protic additive (i-PrOH) also
plays an important role as its presence significantly improves the
yield of 1 and the stereoinduction (entry 4). Lowering or increasing
the amount of CuCl, concentration, the amount of B2pin2 or the
reaction temperature did not positively affect the reaction
efficiency (entries 5-11). Replacing the protic additive by HFIP
(hexafluoroisopropanol, entry 12) gave an intractable reaction
mixture whereas the use of t-amyl-alcohol (entry 13)
demonstrated somewhat lower catalytic potential in terms of yield
of 1 and its E/Z ratio. Further changes in solvent and base (entries
14-18) did also not improve the outcome of this conversion.
Finally, several Cu(I) and Cu(II) precursors were scrutinized
(entries 19-22) showing that both types of precursors allow for
effective catalysis. Compared to CuCl, the utilization of CuCN at
room temperature produced a slightly lower yield of 1 (entry 21),
though at 50 ºC the presence of this precursor led to the best
catalytic performance furnishing the diborylated product in 73%
isolated yield and maintaining high stereocontrol (entry 22 versus
Scheme 3. Influence of propargylic epoxide S0 and supporting ligand in the Cu-
catalyzed diborylation process leading to 1.
The scope of this stereoselective diborylation process
(Scheme 2) was further investigated using a more ample set of
alkynyl cyclic carbonate precursors (S1-S16: for their synthesis,
see the SI).[9] Diborylated compounds 112 were produced with a
high degree of stereocontrol, appreciable yields and reasonable
skeletal variation useful in post-synthetic operations.[11b] Products
1316, however, were isolated in low yields and with substantially
lower levels of stereoinduction which is ascribed to the increasing
steric impediment exerted by the R2 and R3 substituents of the
cyclic carbonate precursor. These combined results (Schemes 2
and 3) clearly show that rather different reactivity and
chemoselectivity is observed in the conversion of the alkynyl-
substituted cyclic carbonates versus epoxides in the presence of
B2pin2 and a protic additive (cf., Scheme 1a).
2
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The nature and influence of the propargylic precursor and the
supporting ligand was further examined (Scheme 3, entry 1). We
were able to reproduce the outcome of the diborylation procedure
reported by Szabó et al.[11a] using a propargylic epoxide (S0)
providing a rather similar yield of 1 (80%) and E/Z ratio for the
isolated product (4:1). The nature of the Cu precursor (entries 3
and 4) had a significant impact on the yield of 1, whereas the
presence of i-PrOH gave 1 in low yield though with high
stereofidelity (entry 4, E/Z>20:1). By applying the optimized
conditions from Table 1 (entry 22: CuCN/ICy·HCl), propargylic
epoxide S0 was converted into diborylated product 1 (50%) with
an E/Z ratio of >20:1 (entry 5). These control experiments
demonstrate that a Cu-carbene catalyst with a protic additive is
able to produce 1,2-diborylated 1,3-dienes with significantly
higher levels of stereoinduction.
product from alkynyl cyclic carbonate S1, which was identified as
the formally reduced -hydroxyallene 17 (67%, Scheme 4). The
yield of 17 did not change (66%) when lowering the reaction
temperature from 50 to 25 ºC, and furthermore only a slight
excess (1.2 equiv) of B2neop2 was required. The reaction could
be easily scaled up (i.e., using 5 mmol of starting carbonate)
delivering 17 in 82% yield (0.77 g).
Scheme 5. Mechanistic control experiments.
With these conditions in hand, we explored the scope of this
unusual borylation driven dichotomy.[14] Various alkynyl cyclic
carbonates (S17-S39) could be conveniently converted into their
(reduced) -hydroxyallenes (Scheme 4, 1836) with ample
diversity in the substitution and functional groups in the presence
of B2neop2 and the same catalyst employed to access the
diborylated 1,3-dienes (Scheme 2, 116). Most of the allene
products were isolated in appreciable yields. Unlike in the case of
diborylation, the presence of more rigid or more sterically
demanding substituents in the carbonate substrate did not pose a
restriction to the formation of the -hydroxyallenes (cf., 2832)
and the developed protocol allows for the presence of various
fragments in the final product including alkyl halides (20, 23, 27
and 34), protected alcohols (cf., 24 and 26) and cycloalkane rings
(29 and 36). Gratifyingly, six-membered alkynyl cyclic carbonate
Scheme 4. Formal reduction of alkynyl cyclic carbonates S17-S39 in the
presence of B2neop2 under similar reaction conditions used in Table 1, entry 22.
All yields are of the isolated product. [a] Reaction time was 20 h.
We next wondered whether the scope of 1,2-diborylated 1,4-
dienes could be further amplified by variation of the diboron(4)
compounds. We considered the use of B2neop2 [bis(neopentyl
glycolato)diboron], as this reagent is commercially available and
has frequently been used in other types of organic
transformations.[2i,13] By applying the same catalyst and reaction
conditions reported for the synthesis of diborylated compounds 1-
16 (Scheme 2), we observed the primary formation of a different
3
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10.1002/anie.202014310
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RESEARCH ARTICLE
precursors (see the SI for more details) were also productive
substrates in this catalytic methodology giving straightforward
access to -hydroxyallenes 3739 in good yields. The use of six-
membered alkynyl carbonates further expands the repertoire of
hydroxyallene scaffolds and validates the great versatility and
synthetic potential of these functionalized cyclic carbonates
compared to the significantly less modular propargylic epoxides.
The remarkable difference in chemoselectivity between the
catalytic experiments performed in the presence of B2pin2 or
B2neop2, despite of their structural similarity, was then scrutinized
in detail through several control experiments and synthetic
extensions (Scheme 5). First, the diborylation of the TMS-
protected alkynyl cyclic carbonate S40 (Scheme 5a) was
attempted. However, the major constituent of the product mixture
turned out to be a mono--borylated cyclic carbonate product
using either B2pin2 or B2neop2. The identity and atom connectivity
stereoselectivity.[17] Interestingly, for these latter conversions
virtually the same catalytic process was effective as the one
leading to the diborylated synthons (Scheme 2) and -
hydroxyallenes (Scheme 4).
of
40a
was
unambiguously
established
by
X-ray
crystallography.[12] This -regioselectivity is believed to be driven
by electronic control, as previously Sun and coworkers described
that silyl substituents act as strong -regio-controlling substituents
in propargylic borylations.[15] This precedent not only allows to
rationalize the -borylation of S40, but also points at the -
borylation being the productive pathway towards 1,3-diborylation.
Intrigued by this observation, we then performed the
borylation of substrate S1 under the optimized reaction conditions
using a slight excess (1.05 equiv) of B2pin2 (Scheme 5b). Four
major components were identified by 1H NMR spectroscopy in the
reaction mixture being unreacted starting material S1 (12%),
diborylated 1,4-diene 1 (15%), -hydroxyallene 17 (45%) and the
mono-borylated allene 41 (28%, cf. -borylation of the alkyne
unit; see SI for details).[16] These data illustrate that an excess of
B2pin2 is required to favor the formation of the diborylated diene,
and that the mono-borylated allene 41 is a likely precursor to 1. In
the absence of a sufficiently large excess of B2pin2 alternative
parasitic pathways may be favored. The formation of -
hydroxyallene 17 as the major compound under these reaction
conditions suggest that alcohol-promoted protodeborylation of 41
is competitive.
Next a competition experiment was carried out adding both
B2pin2 and B2neop2 to a mixture of S1 and the Cu-carbene catalyst
(Scheme 5c). No scrambling of boronate groups was observed for
1, and both 1 (20%) and 17 (45%) could be isolated pointing to
similar order of magnitude kinetics of the first borylation step in
both manifolds. This suggests that the origin of the observed
dichotomy in the process is related to the relative kinetics of the
second borylation step versus protodeborylation. Since the BC
bond in the Bneop functionalized allene intermediate reminiscent
to 41 is sterically more susceptible for transmetalation followed by
protonation, the preferred formation of the formally reduced -
hydroxyallenes (Scheme 4) in the presence of B2neop2 and the
Cu-carbene complex can be rationalized.
The proposed proto-demetalation step induced by the alcohol
additive was examined converting substrate S1 into -
hydroxyallene 42 (Scheme 5d) in the presence of i-PrOD
providing the product with 74% deuterium incorporation being in
line with our mechanistic hypothesis. Finally, the six-membered
alkynyl cyclic carbonate S37 was converted into the silylated -
Scheme 6. The proposed manifolds for 1,2-diborylated 1,3-diene (bottom half)
and -hydroxyallene formation (upper part). L stands for the carbene ligand ICy.
Based on our observations and control experiments, a
mechanistic description of the borylation process is presented in
Scheme 6. After initial formation of a Cu(I)-boryl species (lower
part), the reaction advances with an addition-elimination
sequence that involves the first borylation step (A), carbonate
ring-opening and elimination of the Cu complex providing a
borylated allene with a pendent OCO2Bpin leaving group (B).
Evidence for the intermediacy of B was found by ESI(+)-MS (see
the SI), and its structure resembles generically the one proposed
by Szabó et al.[11a] Subsequently, the allyl-OCO2Bpin species B
will then undergo a second Cu-catalyzed borylation via a SN2´-
type mechanism, after which the Cu-catalyst is regenerated via
protodemetalation. A 1,3-sigmatropic rearrangement involving the
[CuL] complex D allows to explain 1,2-diborylated 1,3-diene
formation from the envisioned productive 3-metaled isomer C.
The requisite for having first a borylation on the -carbon of the
hydroxyallene 43 in 72% yield demonstrating
a useful
amplification of this type of building block,[9] and -hydroxyallene
17 was transformed via a two-step process into the mono-2-
borylated 1,3-diene 45 in good yield and with high
4
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10.1002/anie.202014310
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RESEARCH ARTICLE
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2015, 71, 2183–2197. For some original (recent) contributions: k) N.
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alkyne is supported by the isolation of compounds 40a and 40b
(Scheme 5a), as -borylation would not allow for these envisioned
consecutive steps to take place. An excess (2.5 equiv) of B2pin2
in the proposed manifold agrees well with the experimental
observation of a mixture of compounds when a virtually
stoichiometric amount was present (Scheme 5b). Under these
conditions, the concomitant formation of mono-borylated allenol
41 is noted being a likely precursor to the 1,2-diborylated 1,3-
diene product. The whole process involves two CO bond
scission steps, with the second one leading to an (E)-configured
diborylated alkene under steric control.
In the case of B2neop2 (upper part of Scheme 6), the reaction
takes a different course though the first addition step (E) is
reminiscent to the one (A) based on B2pin2. Upon -oxygen
elimination, a borylated allene carbonate copper species F is
formed that, with the assistance of i-PrOH,[4,18] can undergo a
directing group controlled intramolecular transmetalation (G).[19]
The so-formed copper allene borocarbonate intermediate G
would produce the final -hydroxy allene via a protonation
process while releasing CO2 and neopBOi-Pr, and regenerating
LCu(Oi-Pr) for subsequent turnover. The competition experiment
using a 1:1 molar ratio of both B2pin2 and B2neop2 (Scheme 5c)
and the deuterium labeling experiment (Scheme 5d) are in line
with this scenario, and the apparent easier activation of the
CBneop bond by the Cu complex will lead preferentially to the -
hydroxyallene product.
[3]
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In summary, we have discovered a new dichotomic behavior
of diboron(4) reagents in the Cu-mediated catalytic conversion of
alkynyl cyclic carbonates. This new process significantly
expedites the synthesis of useful 1,2-diborylated 1,3-dienes, and
- and -hydroxy allenes using a mild and simple catalytic
approach. Virtually the same catalytic protocol allows for the
stereoselective conversion of -hydroxyallenes into 2-borylated
1,3-dienes, making the developed catalyst thus a privileged
system for the preparation of wide range of synthetically valuable
synthons.
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a) J. E. Gómez, A. Cristofol, A. W. Kleij, Angew. Chem. Int. Ed. 2019, 58 ,
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Acknowledgements
We thank the Cerca program/Generalitat de Catalunya, ICREA,
MINECO (CTQ2017-88920-P) and AGAUR (2017-SGR-232) for
support. KG thanks the Chinese Research Council for a
predoctoral fellowship (CSC-2017-06920025). Dr. Xisco
Caldentey is thanked for assistance with the HTE screening
phase and Dr. Eduardo Escudero-Adán for the X-ray
crystallographic studies.
[9]
K. Guo, A. W. Kleij, Org. Lett. 2020, 22, 3942-3945.
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thioallenes: g) N. Morita, N. Krause, Angew. Chem. Int. Ed. 2006, 45,
1897-1899.
Keywords: alkyne carbonates • borylation • copper • dichotomy
• homogeneous catalysis
[11] Some of these diborylated products were reported before by Szabó and
[1]
a) M. Burns, S. Essafi, J. R. Bame, S. P. Bull, M. P. Webster, S. Balieu,
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[12] For more details see CCDC-2035473 and 2035474.
[2]
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Entry for the Table of Contents:
It cuts both ways: A mild copper mediated protocol has been developed that allows for dichotomic borylation of alkynyl substituted
carbonates affording either 1,2-diborylated 1,3-dienes or -hydroxy allenes as the principal products depending on the nature of the
diboron(4) reagent. A mechanistic rationale is presented that corroborates with a crucial role for the relative kinetics of the second
borylation step versus i-PrOH assisted protodemetalation.
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